Display devices, such as liquid crystal display devices in which a memory is provided in each pixel in a display area, are known. In this type of display device, a storage period and a display period are alternately repeated. In a storage period, digital data based on the image to be displayed is written to each memory. In a display period, an image is displayed in the display area by setting the drive potential of each pixel to potential corresponding to the digital data stored in a corresponding memory. The system for driving the pixels based on the digital data stored in the memories in the above manner is called, for example, a digital mode or a digital drive system.
In addition to the function of a digital mode, display devices having the function of an analog mode (or an analog drive system) to change the drive potential of each pixel to a multilevel gradation are suggested.
To realize the operations in a storage period and display period, various types of circuits and switching elements are provided in each pixel. When the switching elements are on and off, components such as pixel electrodes are electrically changed to a floating state in a storage period. The potential of the components in a floating state varies according to the change in potential of other components. Because of this variation, an undesired electric field is generated in pixels. In this way, the brightness of images may be changed.
Display devices in which the above digital mode is employed are required to improve the display quality.